Process for producing a brittle



United States Patent No Drawing. Filed Apr. 24, 1961, Ser. No. 104,799 Claims. (Cl. 75-5) The present invention relates to a process for producing a brittle Raney-silver alloy, and more particularly to producmg such an alloy which is brittle and therefore easily grindable.

It is known that catalysts of high activity for various purposes may be produced by chemically or electro chemically dissolving at least one catalytically inactive component from an alloy containing two or more components. For example, Raney-nickel produced in this manner represents an excellent catalyst for hydrogenation reactions. Other well known catalysts in this connection include Raney-cobalt, Raney-iron, Raney-copper, and Raney-molybdenum.

With respect to alloys of the foregoing type, aluminum and zinc are well suited as catalytically inactive components, whereby the known Raney alloys produced therewith may be subjected to activation so as to dissolve out the aluminum or zinc, respectively. Consequently, Raney metal catalysts are achieved which are very effective for hydrogenation reactions. The composition of the Raney alloy is always chosen such that the Raney metal catalyst obtained after the dissolving out of the inactive component or components will possess an optimum of catalytic activity. For this purpose, particular ratios of catalytically active and inactive components are selected whereby the activated Raney catalyst will be most effective. Furthermore, the Raney alloy should be brittle enough that it can be readily ground to fine powder form. Since the brittleness as well as the catalytic activity are both determined by the composition of the alloy, it is necessary in certain cases to bring about a compromise between the requirements of catalytic activity of the Raney catalyst on the one hand and the grindability properties of the alloy on the other hand.

With specific reference to Raney silver, it is particularly difiicult to provide optimum catalytic activity with respect to the Raney silver catalyst and at the same time meet the requirements of grindability of the alloy. While it is true that various silver-aluminum alloys are known which are too brittle to be worked in some metal cutting machines, without regard to especially silver-rich or aluminum-rich limiting phases, such alloys are still so ductile that they can be hardly ground to fine powder form in known grinding devices.

Aside from the objectionable ductile properties of silver aluminum alloys, a Raney silver catalyst can be generally produced from a silver-aluminum alloy, as for example by activating such an alloy having an aluminum content of more than by weight by treatment with concentrated potassium hydroxide solution. A catalyst produced in this manner, for example, possesses outstanding catalytic activity for cathodic reduction of oxygen electrochemically. In this regard, a Raney silver alloy consisting of 65% by weight of silver and 35% by weight of aluminum is especially well suited for such purpose.

It is an object of the present invention to overcome the foregoing disadvantages of silver-aluminum alloys too ductile to be efficiently ground to fine powder form, and to provide a process for producing a brittle easily grindable Raney-silver alloy by rapidly chilling a silver-aluminum alloy melt.

Other and further objects of the invention will become apparent from a study of the within specification and accompanying example.

It has been found in accordance with the present invention that an effective process may be provided for producing a brittle easily grindable Raney silver alloy which comprises rapidly chilling a silver-aluminum alloy melt containing from 45% by weight of silver and 1560% by weight of aluminum, from an elevated temperature to a temperature below degrees C. The chilling is most advantageously carried out from a temperature above 560 degrees C. to a temperature below 100 degrees C. Preferably, the alloy contains 65 by weight of silver and 35% by weight of aluminum.

In accordance with a preferred embodiment of the in vention, the chilling is carried out from a temperature between 700 and 800 degrees C. to a temperature below about 100 degrees C.

In any case, the rapid chilling is preferably effected during a period of only a few seconds whereby to change the alloy from melt phase to brittle easily grindable solid phase. Such rapid chilling may be carried out by quenching the melt with water. Such quenching is desirably provided by pouring the melt directly into water or by immersing the crucible containing the melt into a water bath.

It will be appreciated that in accordance with the present invention, the silver-aluminum alloys obtained by rapidly chilling the melt are much more brittle than slowly cooled alloys of the same composition, and accordingly such brittle solid phase products may be easily and readily ground to fine powder form, as for example, by means of a ball mill.

Alloy powders obtained by such grinding are especially well suited for producing silver double skeleton catalyst electrodes for the electrochemical reduction of oxygen in fuel cells. Such electrodes, for example, are more fully described in co-pending US. application Serial No. 781,- 140, filed December 17,1958.

The following example is set forth for the purposes of illustrating the present invention, but it is to be understood that the present invention is not to be limited thereby. In the example, a comparison is set forth between a silver-aluminum alloy produced by rapidly chilling a melt in accordance with the present invention as compared with 'a silver-aluminum alloy produced in the conventional manner.

Example A Raney silver alloy composed of 65% by weightof silver and 35% by weight of aluminum was melted at 1,000 degrees C. under a protective melt of calcium chloride in a graphite crucible. The melt regulus was allowed to cool slowly for 2 hours to ambient temperature. The solid phase alloy produced in this manner was so ductile that it could not be broken in a press but instead was only pressed into fiat form. In order to obtain a powder thereof, part of the alloy had to be jammed into a turning lathe and subdivided by turning so as to form small pieces of about 0.5 mm. particle size. Such pieces were than ground for 48 hours in a heavy ball mill in the presence of alcohol and water whereby a powder was obtained which consisted essentially of extremely thin, tiny metal plates. Such plates were nearly completely unsuitable for powder metallurgical processmg.

If the same melt containing 65% by weight of silver and 35% by weight of aluminum at 1,000 degrees C. is first tempered at 750 degrees C. and then suddenly poured into water and thereby rapidly chilled to about 40 degrees C., pea-sized globules are obtained. Such globules could be crushed in a press to form little lumps of a particle size of 1 to 2 mm. The latter were ground for hardly 2 hours duration in a ball mill in the manner described above, whereby a fine powder containing nearly spherical particles was obtained.

The particle size of the said powder was less than 100 1.. One part by weight of the alloy powder was intimately mixed with 1.2 parts by weight of carbonyl nickel having a particle size of 3 to 5 16 g. of this mixture of powders was then filled into a cylindrical matrix and compression-molded with a pressure of 1 ton/cm. at 36 C. to form a plate shaped porous electrode body of 40 mm. diameter.

This electrode body was treated with 6-normal KOH- solution in known manner, in order to dissolve the aluminum out of the Raney silver alloy leaving behind Raney silver and thereby to impart to it its large-area double skeleton structure. The electrode thus obtained was used as oxygen diffusion electrode in a fuel cell, it could be permanently cathodically loaded with 400 mm. A./cm. at 50 C.

If a slowly cooled Raney silver alloy, which was subdivided in the manner mentioned above was used instead of the rapidly chilled Raney silver alloy according to the invention, a dense, non porous electrode body was obtained, which could be only superficially attacked by the 6-normal KOH solution. Examination of the electrode body proved, that the tiny plates of the Raney silver alloy became plastically flowing during compression-molding at elevated temperature, thereby completely filling the interstices between the particles of carbonyl nickel, so that the resulting dense electrode body could not be activated by treatment with 6-normal KOH-solution, even if the said activation was performed at nearly boiling temperature of the KOH-solution.

The tiny plates of the slowly cooled Raney silver alloy may be produced in any size. However, since one dimension thereof is diminished to about one tenth in relation to the two other dimensions, the plates adhere to the walls of the ball mill, thereby forming foliated agglomerates, which slow down the grinding process.

What is claimed is:

1. Process for producing a brittle easily grindable Raney-silver alloy which consists essentially of rapidly chilling a silver-aluminum alloy melt containing from 85-40% by weight of silver and -60% by weight of aluminum from an elevated temperature to a temperature below 100 degrees C. during a period of only a few seconds to change said alloy from melt phase to brittle, easily grindable solid phase.

2. Process according to claim 1 wherein said chilling is carried out from a temperature above 560 degrees C. to a temperature below 100 degrees C.

3. Process according to claim 2 wherein said alloy con- 4 tains by weight of silver and 35% by weight of aluminum.

4. Process according to claim 1 wherein said chilling is carried out from a temperature between 700 and 800 degrees C. to a temperature below 100 degrees C.

5. Process for producing a brittle, easily grindable Raney-silver alloy which consists essentially of rapidly chilling a silver-aluminum alloy melt containing from -40% by weight of silver and 15-60% by Weight of aluminum from an elevated temperature to a temperature below C., said rapid chilling being effected by quenching the melt with water.

6. Process according to claim 5 wherein said quenching is carried out by pouring the melt directly into water.

7. Process according to claim 5 wherein said quenching is carried out by maintaining said melt in a crucible and immersing said crucible in a water bath.

8. In the method for improving the brittleness and grindability of a Raney silver alloy, the improvement which consists essentially of rapidly chilling a silveraluminum alloy melt containing from 85-40% by weight of silver and 15-60% by weight of aluminum from a temperature above about 560 C. to a temperature below about 100 C. during a period of only a few seconds to change said alloy from melt phase to brittle, easily grindable solid phase.

9. Improved alloy having enhanced brittleness and grindability properties produced by the process which consists essentially of rapidly chilling a silver-aluminum alloy melt containing from 85-40% by weight of silver and 15-60% by weight of aluminum from an elevated temperature to a temperature below 100 C. during a period of only a few seconds to change said alloy from melt phase to brittle, easily grindable solid phase.

10. Improvement according to claim 9 wherein the alloy contains 65 by weight of silver and 35% by weight of aluminum, the chilling being carried out from a temperature above about 560 C. to a temperature below about 100 C.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Goetzel: Treatise on Powder Metallurgy, vol. 1, 43, Publishers, Inc., New York.

Metals Handbook, 1948 Ed., page 1146. Publ. by the American Society for Metals.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, 126,273 March 24-, 196

Eduard Justi et al.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

In the heading to the printed specification, between line 10 and 11, insert the following:

Claims priority, application Germany Apr., 27, 1960 column 2, line 63, for "than" read then column 3 line for "36 C. read 360 C.

Signed and sealed this With day of July 1964.

(SEAL) Attest:

ESTON G. JOHNSON EDWARD J. BRENNER Attesting Officer Commissioner of Patents 

1. PROCESS FOR PRODUCING A BRITTLE EASILY GRINDABLE RANEY-SILVER ALLOY WHICH CONSISTS ESSENTAILLY OF RAPIDLY CHILLING A SILVER-ALUMINUM ALLOY MELT CONTAINING FROM 85-40% BY WEIGHT OF SILVER AND 15-60% BY WEIGHT OF ALUMINUM FROM AN ELEVATED TEMPERATURE TO A TEMPERATURE BELOW 100 DEGREES C. DURING A PERIOD OF ONLY A FEW SECONDS TO CHANGE SAID ALLOY FROM MELT PHASE TO BRITTLE, EASILY GRINDABLE SOLID PHASE. 